1. Field of the Invention
The present invention relates to a diffraction grating for plural wavelengths, an optical pickup apparatus and an optical disk apparatus capable of handling plural wavelengths.
2. Description of the Related Art
At present, an optical record medium includes a medium such as CD-ROM (Compact Disc-Read Only Memory) or CD-R (Compact Disc-Recordable) capable of recording only one time in which a thickness of a base material of a disk is 1.2 mm and a wavelength of semiconductor laser light used in recording and reproduction is a band of 780 nm (nanometer), or a medium such as DVD-ROM (Digital Versatile Disc-Read Only Memory) or DVD-R (Digital Versatile Disc-Recordable) capable of recording only one time in which a thickness of a base material of a disk is 0.6 mm and a wavelength of semiconductor laser light used in recording and reproduction is a band of 650 nm.
As an optical pickup apparatus for handling such various optical record media, an apparatus dedicated to each the optical record medium has been developed. However, as the optical pickup apparatus, it is desirable to be able to reproduce or record both the optical record media of various different kinds of, for example, CD and DVD by the same apparatus. Also, as an optical disk apparatus, it is probably preferable to be able to reproduce or record various optical record media such as CD or DVD by an apparatus into which one pickup apparatus is built, resulting in cost reduction, savings in space and improvement in performance.
Conventionally, as an optical pickup apparatus for satisfying this kind of requirement, an apparatus in which at least two or more semiconductor laser light sources corresponding to each of the optical record media, separately independent objective lenses and light detection systems are placed inside the same cabinet has been used (see FIG. 6 of JP-A-2001-155375). However, such an optical pickup apparatus becomes large-scale and complicated as compared with a normal optical pickup apparatus dedicated to each disk. Also, a rise in price cannot be avoided.
On the other hand, cost reduction, simplification and miniaturization of an optical pickup apparatus have been implemented with an increase of use of optical record media. As a result of this, also in an optical pickup apparatus capable of handling plural kinds of optical record media, shared use of optical parts and a decrease in the number of parts are required. As one example of solving such a problem, for example, a monolithic semiconductor laser for two wavelengths in which a semiconductor laser with a wavelength band of 790 nm and a semiconductor laser with a wavelength band of 650 nm are formed inside one chip or a semiconductor laser for two wavelengths made of plural chips in which laser chips of each wavelength band are placed so that a distance between light emission points is a distance of about 100 to 300 μm has been proposed as a semiconductor laser for emitting light of two wavelengths.
In the case of using these semiconductor lasers for two wavelengths, the number of parts decreases and miniaturization and cost reduction can be achieved. However, when a diffraction grating used for three beam generation in a differential push-pull method or a three beam method is used in combination with the semiconductor laser for two wavelengths in the optical pickup apparatus, even in the case that any light with a wavelength band of 790 nm for CD reproduction or a wavelength band of 650 nm for DVD reproduction enters the diffraction grating, diffracted light is formed, so that a problem that light quantity loss is caused and signal light decreases arises. Also, extra diffracted light may result in stray light to be mixed into a photodetector and a problem that information cannot be recorded and reproduced arises.
As handling of such problems, a technique for providing diffraction gratings of two kinds is disclosed in JP-A-2001-155375. That is, in JP-A-2001-155375, an optical head apparatus using a hologram for two wavelengths in which two diffraction gratings in which one diffraction grating diffracts light of a wavelength λ1 and transmits light of a wavelength λ2 and the other diffraction grating diffracts light of the wavelength λ2 and transmits light of the wavelength λ1 are provided separately in a direction of the optical axis is disclosed. According to the invention of this JP-A-2001-155375, miniaturization and weight reduction are achieved while recording or reproduction can be performed efficiently and stably.
Also, as one example of solving a problem that unnecessary diffracted light is generated from a diffraction grating, in. JP-A-2001-281432, it is configured so that unnecessary diffracted light is not generated by being constructed so that a phase difference between a protrusion and a depression is 2π in transmitted light of a first wavelength so as to transmit incident light of a first wavelength and diffract second incident light. Also, a technique in which a diffraction grating having diffraction action on incident light of a wavelength λ2 is provided on one surface of a transparent substrate and another diffraction grating having diffraction action on incident light of a wavelength λ1 is provided on the other surface is disclosed in this JP-A-2001-281432.
In the invention described in JP-A-2001-155375, diffraction grating bodies are required every each wavelength and light emitted from a light source passes through plural diffraction grating bodies and light quantity loss increases and also wave front aberration increases. As a result of this, a signal error tends to occur and a problem tends to arise in terms of performance as an optical disk apparatus. In order to avoid this problem, the need to increase a function of a reproduction circuit portion arises, and there arises a disadvantage in manufacturing efficiency and cost. Also, in the invention described in JP-A-2001-155375, the plural diffraction grating bodies are required, so that the number of parts increases and assembly efficiency becomes worse and also cost increases.
Also, according to each the invention of JP-A-2001-281432, for example, as in the case that focusing by a three beam method is adopted in both of the CD apparatus and the DVD apparatus, for an optical system in which a diffracted light component is required every each wavelength, plural diffraction grating bodies described in JP-A-2001-281432 are required or the need to provide depressions for a diffraction grating on both surfaces of one diffraction grating body arises. When plural diffraction grating bodies are provided, a problem that light quantity loss occurs and wave front aberration increases in each the wavelength arises. Also, the number of parts increases and obstacles to miniaturization, simplification and cost reduction are caused.
On the other hand, in the case of providing depressions on both surfaces of the diffraction grating body, light quantity loss does not increase too much and also an increase in wave front aberration is not large, but the light quantity loss or the wave front aberration increases still as compared with the case of having one diffraction grating. Also, the need to provide depressions on both surfaces of the diffraction grating body arises, and the need to adjust its alignment optimally arises. Further, a grating structure having protrusions and depressions on all of both surfaces of the diffraction grating body is provided, so that a coating for increasing transmittance of light is not applied to both the surfaces and a problem that transmittance of light decreases also arises.